Synthetic jet actuators are a widely-used technology that generates a synthetic jet of fluid to influence the flow of that fluid over a surface to disperse heat away therefrom. A typical synthetic jet actuator comprises a housing defining an internal chamber. An orifice is present in a wall of the housing. The actuator further includes a mechanism in or about the housing for periodically changing the volume within the internal chamber so that a series of fluid vortices are generated and projected in an external environment out from the orifice of the housing. Examples of volume changing mechanisms may include, for example, a piston positioned in the jet housing to move fluid in and out of the orifice during reciprocation of the piston or a flexible diaphragm as a wall of the housing. The flexible diaphragm is typically actuated by a piezoelectric actuator or other appropriate means.
Typically, a control system is used to create time-harmonic motion of the volume changing mechanism. As the mechanism decreases the chamber volume, fluid is ejected from the chamber through the orifice. As the fluid passes through the orifice, sharp edges of the orifice separate the flow to create vortex sheets that roll up into vortices. These vortices move away from the edges of the orifice under their own self-induced velocity. As the mechanism increases the chamber volume, ambient fluid is drawn into the chamber from large distances from the orifice. Since the vortices have already moved away from the edges of the orifice, they are not affected by the ambient fluid entering into the chamber. As the vortices travel away from the orifice, they synthesize a jet of fluid, i.e., a “synthetic jet.”
A drawback of existing synthetic jet designs is the noise generated from operation of the synthetic jet. Audible noise is inherent in the operation of synthetic jets as a result of the flexible diaphragm being caused to deflect in an alternating motion, and the natural frequencies of the synthetic jet's various operational modes (structural/mechanical, disk-bending, and acoustic) impact the amount of noise generated during operation.
In operation, synthetic jets are typically excited at or near their mechanical resonance mode(s) in order to optimize electrical to mechanical conversion and so as to achieve maximum deflection at minimal mechanical energy input. While synthetic jet operation is optimized when operated at or near their mechanical resonance mode(s), it is recognized that operating the synthetic jet at certain frequencies can generate a substantial amount of acoustic noise, as the acoustic signature of the device is in part determined by the drive frequency of the device. Typical synthetic jet resonance is in the 100-170 Hz range; however, it is recognized that such a resonance range is within the hearing range of individuals, and thus the operation of the synthetic jet at the resonance frequency generates audible acoustic noise.
It would therefore be desirable to provide a synthetic jet having a mechanical resonance below or above the human receptive band (i.e., hearing range), such as below 30 Hz or above 20 kHz, so as to reduce the acoustic signature of the synthetic jet while not affecting the flow output of the device.